In several species viruses cause cancer. Malignant cells may produce viral antigens and complete viruses. The experiments in this study are designed to elucidate some of the cellular and immune mechanisms which may control the expression of oncogenic viruses. Hybrid cells may be produced by fusion of two different cell lines. Oncogenic viruses may be produced in such hybrid cells although in less numbers than in the parent cell line. Investigations are in progress of hybrid cells made by fusion of a non-virus producing cell with a virus-producing cell to determine the effect of the species of the non-virus producing parent on virus production, the effect of the cell origin - whether embryonic or adult, epithelial or lymphoid - on virus production, and the effect of the enzymatic capabilities of the non-producing parent cell line on virus production. Since viruses may be regarded as non-essential genes, the elucidation of the behavior of normal, non-essential genes in hybrid cells may shed some light on the mechanisms controlling virus production. Immunoglobulin production is being studied in a variety of hybrid cells made between cells which do not produce immunoglobulin and those that do. Finally, there is some evidence that tumor antigens are more immunogenic in hybrid cells than in the parental line. For this reason, hybrid cells have been made between mouse plasmacytomas and other cell lines. Studies are being conducted which will compare the immunogenicity of these hybrids with the parental plasmacytoma.